New zero-day vulnerability details about VirtualBox was revealed by a security researcher, and the researcher has released proof-of-concept videos.
According to the vulnerability description, this flaw affects all VirtualBox version that includes the latest VirtualBox 5.2.20.
A default VirtualBox virtual network device is Intel PRO/1000 MT Desktop (82540EM) and the default network mode is NAT. We will refer to it E1000.
The E1000 has a vulnerability allowing an attacker with root/administrator privileges in a guest to escape to a host ring3. Then the attacker can use existing techniques to escalate privileges to ring 0 via /dev/vboxdrv.
To send network packets a guest does what a common PC does: it configures a network card and supplies network packets to it. Packets are of data link layer frames and of other, more high level headers. Packets supplied to the adaptor are wrapped in Tx descriptors (Tx means transmit). The Tx descriptor is data structure described in the 82540EM datasheet (317453006EN.PDF, Revision 4.0). It stores such metainformation as packet size, VLAN tag, TCP/IP segmentation enabled flags and so on.
The 82540EM datasheet provides for three Tx descriptor types: legacy, context, data. Legacy is deprecated I believe. The other two are used together. The only thing we care of is that context descriptors set the maximum packet size and switch TCP/IP segmentation, and that data descriptors hold physical addresses of network packets and their sizes. The data descriptor’s packet size must be lesser than the context descriptor’s maximum packet size. Usually context descriptors are supplied to the network card before data descriptors.
To supply Tx descriptors to the network card a guess writes them to Tx Ring. This is a ring buffer residing in physical memory at a predefined address. When all descriptors are written down to Tx Ring the guest updates E1000 MMIO TDT register (Transmit Descriptor Tail) to tell the host there are new descriptors to handle.